The presence of acetylocholinesterase in adrenergic nerves has been investigated combining the technique of Karnovsky and Roots for the demonstration of acetylcholinesterase activity with electron microscopic autoradiography for the demonstration of norepinephrine uptake. Our goal was to answer the question as to the existence in the kidney of the two types of fibers one cholinergic (acetylcholinesterase-positive) and other adrenergic (acetylcholinesterase-negative and able to take up norepinephrine). This has been demonstrated in other organs (Graham, J.D.P. et al., Br. J. Pharmac. Chemother. 33: 15-20, 1968; Esterhuizen, A.C. et al., Br. J. Pharmacol. Chemother. 32: 46-56, 1968). Analysis of our results supports the view that acetylcholinesterase is present in the renal adrenergic nerves and therefore the use of this enzyme as a marker for cholinergic nerves in the kidney is questionable. We have continued our investigation of the myelinated nerves of the kidney by serial section microscopy and are extending our investigations of the innervation of the glomerular arterioles by light and electron microscopic autoradiography, to studies of the innervation of other portions of the nephron.